The present invention relates to a granulated sodium perborate product with an active oxygen content over 10% by weight. More particularly, the present invention relates to sodium perborate monohydrate granulated materials with an active oxygen content of approximately 14 to 16% by weight, made by means of spraying an aqueous solution containing hydrogen peroxide and sodium metaborate in a spray dryer and immediately thereafter granulating the primary particles obtained from the spray dryer.
Sodium perborate, a known material, is used both in the form of the so-called tetrahydrate and in the form of the so-called monohydrate in detergent, bleaching and cleaning agents. As used herein, the term "sodium perborates" denotes products with the typical 6-membered ring structure represented by the formula: ##STR1## When x=0, the compound is sodium perborate monohydrate; when x=4, the compound is sodium perborate trihydrate and when x=6, the compound is sodium perborate tetrahydrate. The designations for the various perborates employed above and also in the following description do not correspond to the scientific nomenclature, but they are common forms in the art.
In addition to the active oxygen content, the exterior form, the nature of the surface, the grain size distribution, the bulk density, the shelf life, and the rate of dissolution play an important part for the selection of a perborate for a specific application, e.g. as sole bleaching agent or in combination with activators or peroxycarboxylic acids. Accordingly, many methods are directed toward influencing these parameters by means of the method of preparation.
Prior art detergent materials have used sodium perborate tetrahydrate formulations almost exclusively. Typical commercially available sodium perborate tetrahydrates products (x=6 in the formula above) have the following physical and chemical properties:
Appearance--White, odorless, freely flowing salt PA1 Active Oxygen Content (Oa)--10.0%-10.4%, PA1 Dissolution Rate (2g/l, 15.degree. C.)--6-8 minutes PA1 Apparent Density--0.7-0.8 kg/l PA1 (bulk density of bulk weight)
determined by titration with potassium permanganate in sulfuric acid.
Characteristics--Tends to agglomeration; over 40.degree. C. PA0 Melting point of technical product--approximately 60.degree. C.
Sodium perborate tetrahydrate is normally prepared by reacting hydrogen peroxide with sodium metaborate in aqueous solution followed by crystallization. The active oxygen content of the tetrahydrate products approaches the theoretical maximum of 10.4% for the tetrahydrate product. Sodium perborates with an available oxygen content (O.sub.a) of over 10.4% by weight, especially sodium perborate monohydrate with an O.sub.a content of approximately 15 to 16% by weight, are obtained either by the dehydration of perborate tetrahydrate or by drying a finely distributed aqueous solution containing hydrogen peroxide and sodium metaborate. Monohydrates with at least 15% active oxygen content are available, which is approaching the theoretical maximum value of 16% for the monohydrate product. In principle, sodium perborate monohydrates can be employed for the same purposes as the tetrahydrate products. The monohydrate product is even more versatile because of its higher active oxygen content and non-caking tendency.
The preparation of sodium perborate monohydrate by means of dehydration of sodium perborate tetrahydrate is practiced on an industrial scale, but this process is expensive because two stages are required, namely, the preparation and isolation of the tetrahydrate and its dehydration. The dehydration conditions must be adapted to the desired properties of the perborate monohydrate, whereby the energy efficiency and/or the space-time yield are unsatisfactory. Commercially available sodium perborate monohydrate manufactured in this manner usually has an O.sub.a --content of 15-16%, an apparent density of 550 g/l and a dissolution rate of less than 1 minute (2 g/l, 15.degree. C.).
The attempt has therefore already been made to obtain sodium perborate with an available oxygen content over 10.4% by weight directly, that is, without first preparing the perborate tetrahydrate.
DE-AS 17 92 273 teaches a method for the preparation of alkali perborates, especially of products low in water of crystallization with correspondingly high active oxygen contents, by reacting suitable alkali compounds with a carrier for active oxygen and spray drying, whereby the active oxygen carrier is added to the spray batch immediately prior to the atomization. An aqueous solution containing hydrogen peroxide and sodium metaborate is preferably sprayed and dried. Products prepared according to this method with an O.sub.a content around 15% by weight are generally in a very finely powdered form. Consequently, their use in detergents is problematical due to the unavoidable formation of dust. The material parameters no longer satisfy the requirements being placed on perborates today.
The method of DE-OS 35 05 158 is based on the same principle, namely, the spray drying of a solution or suspension containing H.sub.2 O.sub.2 and sodium metaborate. The concentration of borate, calculated as B.sub.2 O.sub.3, in the spray batch is limited to 90-130 g/l and the molar ratio of H.sub.2 O.sub.2 to B.sub.2 O.sub.3 is 2.0:1 to 4.2:1. A sodium perborate with an active oxygen content of over 16% by weight is obtained at an outlet air temperature in the spray dryer at 55.degree. to 120.degree. C., preferably 60.degree. to 95.degree. C.
The particle size distribution is particularly in a range of 10 to 20 micrometers; furthermore, the particles are amorphous; the bulk weight is in a range of 0.2 to 0.4 g/ml. As was the case in the older methods practiced in the past, the fineness of the product hampers its applicability for many purposes. In particular, the rate of dissolution is considerably reduced in comparison to the perborate monohydrate; the particles remain suspended in water or even collect on the surface. These are properties which raise a question as to trouble-free use as bleaching agents in detergents.
DE-PS 26 50 225 teaches a method of preparing granulated sodium perborate monohydrate wherein a 5 to 75% by weight aqueous solution containing hydrogen peroxide and a 5 to 40% by weight aqueous solution containing sodium metaborate are sprayed into a fluid-bed drier containing nuclei at a fluid-bed temperature of up to 100.degree. C., and the water is evaporated with the swirling gas. In this method, the product generated in the fluid bed must be sieved, even mechanically comminuted, if necessary, and the perborate monohydrate with dimensions smaller than that which to be prepared is recycled. The high expenditure of energy, the unfavorable particle size distribution obtained at first, which requires a comminution, as well as the constant recycling of nuclei are considered disadvantageous.
If very fine perborates, produced e.g. according to DE-AS 17 92 273 or DE OS 35 05 158, are used as a starting point, granulation methods for building up more coarse products which can be used in detergents can be considered. However, success can not be predicted, especially in the granulation of perborates. Thus, perborate tetrahydrate with a particle size of 0.05 to 0.3 mm was granulated by means of mechanical action of the particles on each other in the presence of adhering water and optional binders in DE-PS 10 37 432. However, a heat treatment had to follow and preferably binders with an "explosive" action had to be used in order to disadvantageously affect the rate of dissolution by the granulation. This patent also points out the possibility of using other perborate hydrates. Nevertheless, examples or even suggestions are not given as to how one can convert sodium perborate monohydrate with an average particle diameter of approximately 5 to approximately 50 micrometers in an industrially simple manner into granulated materials with satisfactory abrasion resistance and a high rate of dissolution.
U.S. Pat. No. 4,002,434 shows treatment of sodium perborate monohydrate wherein the sodium perborate monohydrate is moistened with water or aqueous solutions at 20.degree.-90.degree. C. and dried in a fluid-bed drier, a device customary for granulations. This method does improve the abrasion; however, this patent contains no description of granulation occurring at the same time.
DE-PS 26 52 488 teaches a method of preparing granular materials from zeolites and oxygen-releasing compounds. Sodium peroxoborate is also mentioned; however, it is used as a moist product obtained from the centrifuge. This document yields no suggestions about the preparation of spray-dried, amorphous perborates and their granulation. Moreover, the products according to DE-PS 26 52 488 limit the usage of the peroxoborate contained in them.
Due to the unsatisfactory properties of the sodium perborates available by spray drying, as well as due to the problems to be expected with a separate granulation of very fine particles which are first isolated, it was not obvious to combine these method steps with one another in expectation of obtaining granulated sodium perborate which can be used with success in detergents and bleaching agents.